Field of the Invention
The present invention concerns a method to determine a subject-specific B1 distribution of an examination subject in a measurement volume in a magnetic resonance apparatus, as well as a magnetic resonance apparatus and an electronically readable data storage medium to implement such a method.
Description of the Prior Art
Magnetic resonance (MR) is a known technique or modality with which images of the inside of an examination subject can be generated. Described in a simplified manner, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (called a B0 field) with a field strength of 0.2 to 7 Tesla or more, that causes nuclear spins in the subject to orient along the basic magnetic field. For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field. Radio-frequency (RF) excitation pulses (RF pulses) are radiated into the examination subject to trigger nuclear magnetic resonances.
The flux density of the radiated RF pulses is typically designated with B1. The pulse-shaped radio-frequency field is therefore generally also shortened to “B1 field”. The nuclear spins of the atoms in the examination subject are thereby excited by these radio-frequency pulses such that they are deflected out of their steady state (parallel to the basic magnetic field B0 ) by what is known as an “excitation flip angle” (also shortened to “flip angle”). The nuclear spins then precess around the direction of the basic magnetic field B0. The magnetic resonance signals that are thereby generated are acquired by radio-frequency reception antennas (reception coils). The acquired measurement data (also called k-space data) are digitized and stored as complex numerical values (raw data) in a k-space matrix. By means of a multidimensional Fourier transformation, an associated MR image can be reconstructed from the k-space matrix populated with values. In addition to anatomical images, spectroscopy data, movement data or temperature data of an examined or, respectively, treated area can also be determined with the aid of the magnetic resonance technique.
The measured signals thus also depend on the radiated RF pulses. Typical methods for reconstruction of image data sets from magnetic resonance signals assume a homogenous RF field distribution (B1 field distribution) in the examination volume in addition to a homogenous basic magnetic field and strong linear gradient magnetic fields. In real MR systems, however, the B1 field distributions typically vary spatially in the examination volume, which leads to image inhomogeneities (image artifacts) in the MR images reconstructed from the signals, and therefore to a poorer ability to detect the imaged examination subject. Particularly in the case of whole-body imaging or acquisitions of the torso (chest, abdomen, pelvis), artificial shadows in the image occur due to an inhomogeneous RF field distribution at basic magnetic fields of 3 Tesla or more.
Furthermore, an optimally precise knowledge of the B1 fields present in the examination subject is essential for many applications of magnetic resonance tomography, for example for the pulse calculation in multichannel transmission operation or for quantitative T1 examinations. Due to the subject-specific conductivity and susceptibility distributions, spatially dependent variations of the B1 field that are pronounced in high static magnetic fields (in particular at 3T or more) can occur. Therefore, a subject-specific determination of the actually present B1 distribution at a set transmission power is necessary for many applications.
Methods are known to determine the subject-specific B1 distribution. For example, a method in which the B1 distribution is determined from the flip angle distribution is described in the article by Cunningham et al., “Saturated Double-Angle Method for Rapid B1+ Mapping”, Magn. Reson. Med. (2006) 55: Pages 1326-1333. However, this method is not sufficiently precise for specific flip angles (for example 90°). The acceleration of the method that is described in the article leads to a sensitivity to resonance shifts and limits the dynamic range of the method. Without the acceleration, the method requires a long measurement time.
In “Rapid B1+ Mapping Using a Preconditioning RF Pulse with TurboFLASH Readout”, Magn. Reson. Med. (2010) 64: Pages 439-446, Chung et al. describe a method to determine the B1 distribution in which a slice-selective preconditioning RF pulse is radiated and that is measured via this reduced longitudinal magnetization from which the B1 distribution is determined. Although this method is relatively fast, it is sensitive to the distribution of the T1 relaxation times in the examination subject, which are not always sufficiently known.
In “Actual Flip-Angle Imaging in the Pulsed Steady State: A Method for Rapid Three-Dimensional Mapping of the Transmitted Radiofrequency Field”, Magn. Reson. Med. (2007) 57: Pages 192-200, Yarnykh describes a method in which an AFI pulse sequence (AFI: “actual flip angle”) is used which comprises two identical RF pulses followed by two different wait times TR1 and TR2 that respectively generate an echo signal following each RF pulse. The current flip angles of the RF pulses (and therefore the B1 distribution) can be calculated with the aid of the generated and measured echo signals. However, as a three-dimensional (3D) method this method is particularly sensitive to movements.
Another method in order to determine the spatially dependent B1 field amplitudes (thus the B1 distribution) of an examination subject is the utilization of the Bloch-Siegert phase shift as it is described by Sacolick et al. in “B1 Mapping by Bloch-Siegert Shift”, Magn. Reson. Med. (2010) 63: Pages 1315-1322. The Bloch-Siegert phase shift arises via radiation of a non-resonant RF pulse (called a Bloch-Siegert pulse in the following). The generated phase shift is thereby proportional to the square of the B1 amplitude:
                              ϕ          BS                =                                                            (                                  B                  1                  peak                                )                            2                        ⁢                                          ∫                0                T                            ⁢                                                                                          (                                              γ                        ⁢                                                                                                  ⁢                                                                              B                            1                            norm                                                    ⁡                                                      (                            t                            )                                                                                              )                                        2                                                        4                    ⁢                    π                                                  ⁢                                  dt                  ·                                      1                                          v                      RF                                                                                                    =                      k            ⁢                                          1                                  v                  RF                                            .                                                          (        1        )            
γ designates the gyromagnetic ratio, B1norm(t) designates the time curve of the Bloch-Siegert pulse normalized to the amplitude B1peak, and νRF designates the difference of the frequency of the Bloch-Siegert pulse relative to the resonance frequency. The integration extends over the entire pulse duration; here the frequency of the RF pulse is assumed to be temporally constant. In the underlying method here, the amplitude B1peak of the Bloch-Siegert pulse is calculated from the measured phase shift φBS for a selected transmission power. A schematic representation of a sequence for the Bloch-Siegert method in gradient echo realization as in the cited article by Sacolick et al. is shown in FIG. 1.
In “RF Pulse Optimization for Bloch-Siegert B1+ Mapping”, Magn. Reson. Med. (2011) DOI: 10.1002/mrm.23271, Khalighi et al. describe a method in which resonance shifts due to spatially dependent inhomogeneities of the static B0 field, due to chemical shift and due to susceptibility differences are taken into account and corrected in the calculation of the B1 amplitude using a pre-generated B0 map. However, inaccuracies and errors of the B0 map hereby also propagate in the calculation of the B1 amplitude.